The present invention relates to pressure sensing transducers. More particularly, the present invention relates to pressure sensing transducers of the piezoresistive type.
Piezoresistive pressure transducers have a wide range of applications in any industry where accurate pressure monitoring is required. Typical industrial applications include process monitoring, rotating machinery monitoring and testing, oil exploration, and jet and gas turbine engine controls. Piezoresistive pressure transducers offer many potential advantages in such applications due to their small size, absence of moving parts and potential for sensitivity and accuracy.
The heart of a piezoresistive pressure transducer is a pressure force collector diaphragm having one or more piezoresistive elements mounted thereon. The diaphragm with the piezoresistive elements is typically placed in a pressure cell of some type which maintains a low pressure or vacuum on one side of the diaphragm and allows the external medium under pressure to contact the other side of the diaphragm. A voltage is placed across the piezoresistive element(s) and as the diaphragm bends in response to pressure changes, a resistance change in the piezoresistive element(s) results in a change in the current flowing through the piezoresistive element(s).
There exists an application in the plastic industry known as the polymer melt process where accurate pressure and temperature measurements are essential to control the process. The pressure and temperature typically reach up to 15,000 psi and to 800.degree. F. and above. The external media being measured is typically a slurry, viscous fluid which has corrosive and abrasive properties and is maintained at high temperatures of up to 800.degree. F. As a result, conventional alloys of steel and stainless steel exposed to such media are readily abraded and degraded.
In order to accurately measure the pressure and temperature of such media, however, the piezoresistive elements need to be in intimate contact with the diaphragm deflected by the media. The various components of the pressure transducer must also be able to withstand the high temperatures associated with the polymer melt process. A flush mount force collecting diaphragm capable of operating under such high temperatures with its pressure and temperature sensing elements integrated on the diaphragm would be ideal. However, to the best of Applicant's knowledge there is no known pressure transducer available in the industry which has components which can withstand the high temperature corrosive materials used in the polymer melt process and can provide accurate pressure and temperature measurements directly. As a result, the industry has attempted to develop other pressure transducers that do not locate the piezoresistive elements on the diaphragm which makes intimate contact with the corrosive and abrasive media.
In one approach, a stainless steel force collector diaphragm of approximately 0.005 inch thickness and 0.320 inch diameter having no pressure or temperature sensing elements mounted thereon is arranged so that it is in intimate contact with the media. This diaphragm is strictly a force collector with no sensing capabilities. The force collected through the diaphragm from the media, due to diaphragm deflection, is transmitted through a tube or a capillary filled with mercury to a conventional pressure transducer positioned a safe distance from the external media.
This approach has several distinct disadvantages. First, the thin stainless steel diaphragm is susceptible to abrasion that gradually alters its pressure sensitivity properties, thereby compromising its measuring accuracy. Second, the abrasion of the diaphragm coupled with its exposure to corrosive media may eventually cause the rupture of the diaphragm. This is obviously an extreme hazard when the diaphragm deflection is transmitted to the pressure transducer through a tube filled with a poisonous fluid such as mercury. The risk of mercury contamination is particularly critical in applications where the medical and food plastic products are being manufactured, such as the extrusion of food for human and animal consumption, such as cereals, dog food, etc. Such systems are also more complicated, costly and inconvenient to assembly. Such systems are also less accurate since they do not permit direct pressure and temperature measurement of the media.
In another approach, the mercury is replaced with liquid sodium potassium. Although this eliminates mercury contamination or poisoning hazards, it introduces an entirely new problem of a creating a fire hazard since liquid sodium potassium will spontaneously ignite upon exposure to air when the diaphragm ruptures.
In still another approach, a stainless steel extended push rod is used to transmit the force collected by a diaphragm having no sensing capabilities to the conventional pressure transducer. This approach eliminates the previous problems associated with the fluid filled tube, but creates a new set of limitations to the pressure transducer. It greatly increases the cost of manufacturing the pressure transducer. Moreover, it compromises the pressure measuring accuracy of the pressure transducer and fails to facilitate any media temperature measurement. Accordingly, there is a present need for a pressure transducer which has piezoresistive sensing elements mounted on a diaphragm which makes intimate physical contact with corrosive and abrasive materials and can accurately measure such materials over wide pressure and temperature ranges.